The present invention relates to a light waveform changing apparatus and, more particularly, to a novel light waveform changing apparatus that is suitable for use in optical communications using optical fibers and which is capable of shaping the waveform of an optical signal at high speed without employing an electro-optical converter.
One of the problems encountered in optical communications, in particular, those using optical fibers is that the waveform of a pulse signal being transmitted over a long distance becomes less sharp on due to various factors including wavelength dispersion in optical fibers. In addition, the intensity of a pulse will decrease due to light attenuation. In order to deal with this situation, repeaters must be installed every predetermined distance over the transmission path so that the blunt and attenuated pulse signal is reshaped and amplified into a pulse waveform having sharp leading and trailing edges.
A conventional light waveform shaping apparatus that has been used for this purpose typically has the composition shown in FIG. 22. In this apparatus, an optical signal having a blunt waveform that is launched from an optical fiber 10 is first supplied to a photoelectric converter 12 where it is converted into a electrical signal. The output of the converter 12 is then supplied to a waveform shaping device 14 (e.g. an emitter-coupled logic circuit (ECL), which is a typical high-speed logic circuit) where the waveform is shaped electrically using a predetermined threshold level. Subsequently, the shaped electrical signal is supplied to an electro-optical converter 16 (e.g. a semiconductor laser or LED) where it is restored to an optical signal to be inputted to an optical fiber 18.
The conventional light waveform shaping apparatus described above needs to perform not only photoelectric conversion but also electro-optical conversion, so it is susceptible to waveform distortion and deterioration in response speed during the electro-optical conversion. A further problem with this apparatus is that even if a high-speed ECL is used as the light waveform shaping device 14, each of the rise and fall times of a pulse signal cannot be made shorter than the order of 150 picoseconds. A large signal delay time is another problem. Furthermore, there occurs a coupling loss between the electro-optical converter 16 and the optical fiber 10. Besides these problems, the conventional apparatus requires a power to be supplied to each of the photoelectric converter 12, waveform shaping device 14 and the electro-optical converter 16, resulting in increased power consumption.